Electrically conductive, thermoplastic polymer compounds are of increased practical interest, for instance, for packaging electronic instruments and parts, and to solve a wide range of static discharge, electrostatic dissipation and electromagnetic shielding problems. Often, such compounds are made by mixing, for example, carbon black, stainless steel fibers, silver or aluminum flakes or Nickel-coated fibers with insulating bulk thermoplastics such as polystyrene, polyolefins, nylons, polycarbonate, acrylonitrile butadiene styrene co-polymers (ABS), and the like. These filled compounds are subsequently processed into desired shapes and articles by common plastics processing methods such as extrusion, injection or blow molding and the like. Major problems related to the above filled thermoplastic compounds are that processing of these materials is not trivial and is often associated with excessive machine wear, and that the final compounds frequently exhibit undesirable mechanical properties such as brittleness and a reduced elongation to break in comparison with the corresponding properties of the untilled matrix polymer.
More recently, there has been an increased interest in replacing such carbon black or metal-filled compounds with intrinsically electrically conductive polymers and their blends with common insulating polymers. The latter systems are believed to be more cost competitive, easier to process and to exhibit desirable mechanical properties. Among the various conductive polymers, the polyanilines in particular have attracted attention because of their excellent environmental stability and their low production costs.
Polyaniline is well known in the art, and the preparation of the electrically conductive form of this polymer based on, for example, contacting polyanilines with protonic acids has been described. Green, A. G., and Woodhead, A. E., "Aniline-black and Allied Compounds, Part 1," J. Chem. Soc., Vol. 101, pp. 1117 (1912); Kobayashi, et al., Electrochemical Reactions . . . of Polyaniline Film-Coated Electrodes, J. Electroanl. Chem., Vol. 177, pp. 281-91 (1984); U.S. Pat. Nos. 3,963,498, 4,025,463 and 4,983,322. Typical examples of such described protonic acids are HCL, H.sub.2 SO.sub.4, sulfonic acids of the type R.sub.1 --SO.sub.3 H, phosphoric acids, etc. Chiang, J. -C. and MacDiarmid, A. G., "Polyaniline: Protonic Acid Doping of the Emeraldine Form to the Metallic Regime", Synthetic Metals, Vol. 13, p. 196 (1986); Salaneck, W. R. et al., "A Two-Dimensional-Surface "State" Diagram for Polyaniline", Synthetic Metals, Vol. 13, p. 297 (1986). Such acids form complexes with polyaniline, which, generally, exhibit electrical conductivities of 10.sup.-3 S/cm or more. Their electrical properties make these so-called "doped" polyanilines and their blends and compounds with common insulating bulk polymers suitable for a variety of the antistatic and shielding applications that are currently served by metal or carbon black filled systems.
Processing of polyanilines has been described in several patents and patent applications. In U.S. Pat. No. 5,006,278 a conductive product is described which has been made by mixing a solvent, a doping agent and a polyaniline, whereafter the solvent has been removed by evaporation. In PCT Publication No. WO 9013601 a polymer mixture is prepared by mixing a suitable solvent with a mixture of polyaniline and a multi-sulphonic acid, used as a doping agent, whereafter the solvent is evaporated. According to this specification, the doping is generally carried out at 20.degree.-25.degree. C. it is described that the doping can be carried out as a heterogeneous reaction, followed by dissolution of the mixture in a suitable solvent. The processing into a final shape is carried out in the presence of a solvent. (p. 15, 1.23).
PCT Publication No. WO 9010297, U.S. Pat. No. 5,002,700 and European Patent Publication No. EP 152 632 describe the use of dodecylbenzene sulphonic acid as a doping agent for polyaniline, PCT Publication No. WO 9001775 describes a multi-sulphonic acid as a doping agent for polyaniline with the advantage of better thermal stability compared with other sulphonic acids. In the examples of this specification, the doping of polyaniline has been carried out in a suspension of polyaniline and the sulphonic acid in an aqueous solution of formic acid. In none of the examples of the above mentioned patent specifications, however, have adequate methods been described for melt processing of polyaniline compositions.
Melt processing of compounds comprising conductive polyanilines, has typically been executed by mechanically mixing the components, where the conductive polyaniline is in the solid form and the matrix polymer in its molten form before shaping the blend into the desired article. Generally, the blends obtained exhibit varying conductivity, are of non-homogeneous quality and of poor mechanical properties and, generally show a high percolation threshold for the onset of electrical conductivity.
It has indeed been suggested, that certain polyaniline-based systems and blends may be processed using standard polymer processing techniques. For example, in Plastics Technology 37 (1991):9 pp. 19-20 is described the use of protonated, conductive polyanilines to impart conductivity to mixtures with common insulating thermoplastic polymers such as nylons and poly(vinylchloride). In this application, however, the conductive polyaniline is in the form of solid, intractable particles, which, much like carbon black, are dispersed in the non-conducting matrix, which is in its molten form. Melt-processing of these compounds requires special melt dispersion techniques; European Patent Publication Nos. EP 168 620 and 168 621. A relatively high content of conductive polyaniline is required to reach desirable levels of conductivity in the blends with insulating polymers; or, in other words, the percolation threshold for the onset of conductivity is relatively high. Thus, in the aforementioned blends of solid polyaniline particles dispersed in poly(vinylchloride) a percolation threshold existed of about at least 13% w/w of the conductive polyaniline. Such a high content of conductive polyaniline particles is not desirable, because it is not economical and, in addition, may substantially alter the mechanical properties of the blend in comparison with those of the pure matrix polymer.
An improved method of making conductive polyaniline compositions has been described in Finnish Patent Application 915 760. According to this application, polyaniline, or derivatives thereof, and an excess of an organic protonic acid are mechanically mixed. The liquid-like mixture or suspension is subsequently thermally solidified between 40.degree.-250.degree. C. in a mixer. As a result, a dry, solid composition, in the form of a granulate comprising protonic acid-doped polyaniline is obtained. The solid polyaniline-protonic acid complex can subsequently be mixed with insulting thermoplastic polymers and formed into parts of desired shapes using standard polymer melt-processing techniques. Improved visual surface characteristics and lower percolation thresholds for the conductivity were observed for parts made according to this method. However, as stated above, an excess of acid is used in the technique. The latter generally is unacceptable from a processing, application and environmental point of view. The excess acid, of course, can be removed, but this process is tedious and uneconomical, and limits the scope of the products that can be manufactured.
In a publication in Synthetic Metals (Vol. 48 (1992) pp. 91-97) are described blends that exhibit much lower percolation thresholds, sometimes even below 1% w/w, of conductive polyaniline and a wide variety of non-conducting matrix polymers, such as polyethylenes, poly(vinylchloride), polystyrene, nylons, and the like; poly(methylmethacrylate), polycarbonate, acrylonitrile butadiene styrene copolymers (ABS), and the like. However, invariably, the compositions that exhibit such low percolation thresholds are made from solutions of the conductive polyaniline and the matrix polymers, which is uneconomical, and limits the use to fabrication of products such as film, coatings and fibers.
Thus, clearly, a need exists for electrically conductive polyaniline compositions that can be processed from the melt, and for polyaniline blends with, for example, insulating bulk polymers that exhibit a low percolation threshold, and do not contain an excess of protonic acid, i.e. are neutral or only slightly acidic, and for economical methods to fabricate articles from the melt of such compositions.